This invention relates to structures for field effect transistors (FETs) and, more particularly, to the topology of such structures.
Research and development resources are being directed toward developing gallium arsenide structures because of the potentially high frequency and high density characteristics of these devices. Presently available gallium arsenide FETs utilized in monolithic microwave integrated circuits (MMICs) are of at least two basic types. The interdigitated type utilizes gate fingers which are fed from the ends thereof and provide a total gate width by paralleling as many fingers as desired. Another type utilizes a continuous gate line that is fed from one or more central points or pads. This latter configuration is generally limited to applications requiring lower gain and hence smaller gate widths than the interdigitated type. Each type has a continuous active area of semiconductor material.
Generally it is desired that structures operating at microwave frequencies be symmetrical to facilitate coupling, compactness and even field distributions to thereby reduce loss and other problems. Neither of the foregoing prior art FET structures are symmetrical about both axes. Moreover, such structures generally have limited high power operation because the continuous active areas thereof can only dissipate a limited amount of heat. Furthermore, such structures may require gate fingers of excessive "width" which tend to increase undesirable parasitic effects that result in increased source, drain and/or gate resistances, capacitances and/or inductances. Gate "width" is defined as the total "length" of the gate finger contact portions superimposed on active semiconductor material between source and drain electrode regions.
Furthermore, prior art structures sometimes use "air bridges" which are conductors elevated on posts that connect a plurality of active source regions to a source bus, for instance. These air bridges extend over the active drain regions in some prior art devices Hence, such structures tend to undesirably increase the drain-to-source capacitance and thereby limit the high frequency capabilities of prior art FETs. Moreover, these air bridges also undesirably tend to increase the source inductance, for instance. Additionally, the prior art air bridges undesirably tend to decrease the drain-to-source voltage breakdown characteristics.